Are all fossil animals dinosaurs?No. Dinosaurs are a group of ancient reptiles that had a set of particular skeletal features. These features separated dinosaurs from other ancient reptiles such as Dimetrodon, the plesiosaurs, and pterosaurs. Fossil mammals, such as mammoths and "saber-toothed tigers" are often incorrectly called dinosaurs.

Did people and dinosaurs live at the same time?No. After the dinosaurs died out, over 60 million years passed before people appeared on Earth. Many scientists who study dinosaurs support the hypothesis that birds are closely related to one line of carnivorous dinosaurs, and some consider that they in fact represent modern living species of dinosaurs. This theory is still being debated.

Where did dinosaurs live?Dinosaurs lived on all of the continents. At the beginning of the age of dinosaurs (during the Triassic period, about 230 million years ago) the continents we now know were arranged together as a single supercontinent called Pangaea. During the 165 million years of dinosaur existence, this supercontinent slowly broke apart.

Did all dinosaurs live together and at the same time?Dinosaur communities were separated by both time and geography. The "age of dinosaurs" (the Mesozoic era) included three consecutive geologic time periods (the Triassic, Jurassic, and Cretaceous periods). Different dinosaur species lived during each of these three periods. The Jurassic dinosaur Stegosaurus had already been extinct for approximately 80 million years before the appearance of the Cretaceous dinosaur Tyrannosaurus. At the beginning of dinosaur history (the Triassic period), there was one supercontinent on Earth (Pangaea). Many dinosaur types were widespread across it. However, as Pangaea broke apart, dinosaurs became scattered across the globe on separate continents, and new types of dinosaurs evolved separately in each geographic area.

How are dinosaurs named?Dinosaurs generally are named after a characteristic body feature, after the place where they were found, or after a person involved in the discovery. Usually the name consists of two Greek or Latin words (or combinations); in order, these are the genus (plural genera) and the species name. For example, the Greek and Latin combination (binomen) Tyrannosaurus rex means "king of the tyrant lizards." Biologists name modern animals exactly the same way. Some examples include domestic dogs (Canis familiaris), golden eagles (Aquila chrysaetos), box turtles (Terrapene carolina), and rattlesnakes (Crotalus horridus).

What was the biggest dinosaur? What was the smallest?The largest complete dinosaur we know of was Brachiosaurus ("arm lizard"); it reached 23 m in length and 12 m in height (about the length of two large school buses and the height of a four-story building). Fragmentary leg bones and vertebrae of even larger dinosaur species are known, but these skeletal remains are too incomplete to determine their exact size. Several of these might have been one and a half to two times larger than Brachiosaurus. The smallest dinosaurs were just slightly larger than a chicken; Compsognathus ("pretty jaw") was 1 m long and probably weighed 2.5 kg. If birds are advanced dinosaurs, then the smallest dinosaur would be the hummingbird.

How many types of dinosaurs are known?Approximately 700 species have been named. However, a recent scientific review suggests that only about half of these are based on almost complete specimens that can be shown to be unique and separate. Even if all of the roughly 700 published species are valid, their number is still less than one-tenth the number of currently known living bird species and less than one-fifth the number of currently known mammal species.

Were dinosaurs able to regulate their body temperature?Scientists have conflicting opinions on this subject. Some paleontologists think that all dinosaurs were able to regulate their body temperature in the same sense that modern birds and mammals are; that is, they had rapid metabolic rates. Other scientists think it unlikely that any dinosaur could have had a rapid metabolic rate. The problem is that it is hard to find evidence that unquestionably shows what dinosaur metabolisms were like.

How long could a dinosaur live?Dinosaur life spans probably varied in length from tens of years to hundreds of years. Their possible maximum age can be estimated from the maximum life spans of modern reptiles, such as the 66-year life span of the common alligator and the impressive life span of a black Seychelles tortoise. One specimen of this now-extinct tortoise lived 152 years in captivity and had an accidental death. These estimates, based on life spans of cold-blooded animals, would be too long if dinosaurs had metabolisms more similar to modern birds and mammals.

What did dinosaurs eat?The teeth of dinosaurs reveal that they ate plants or meat. Some hunted other dinosaurs or scavenged dead animals. Most, however, ate plants (but not grass, which hadn't evolved yet). Some scientists think that some dinosaurs ate eggs.

Did dinosaurs communicate?Dinosaurs probably communicated both vocally and visually. The chambered headcrests on some dinosaurs might have been used to amplify grunts or bellows.

Why did some dinosaurs grow so big?Paleontologists don't know for certain, but perhaps a large body size protected them from most predators, helped to regulate internal body temperature, or let them reach new sources of food (some probably browsed treetops, as giraffes do today).

Which was the smartest dinosaur?Although there is no direct way to measure a dinosaur's intelligence, one of the few possible measures of intelligence might be a large brain in a small body. The genus that perhaps fits this description best was the small and agile Cretaceous dinosaur Troodon, which also may have had stereoscopic vision and excellent eyesight and was built for speed. Even so, this dinosaur was probably not as "intelligent" as most modern mammals.

Were dinosaurs social animals?Some dinosaurs were social creatures. Recently discovered evidence indicates that they traveled together and that some may even have migrated. Grouped hadrosaur nest sites have been found with badly crushed eggshells and skeletons of baby dinosaurs still in the nests, suggesting that some babies stayed in their nests after hatching and probably were fed by parents.

When did dinosaurs become extinct?Dinosaurs became extinct about 65 million years ago after living on Earth for about 165 million years. If all Earth time from the very beginning of dinosaurs to today were compressed into 365 days (one calendar year), the dinosaurs appeared January 1 and became extinct the third week of September, or about the first day of autumn. By comparison, people have been on Earth only since December 31 (New Year's Eve).

Why did the dinosaurs die out?There are dozens of theories to explain a probable cause or causes. Throughout the Mesozoic era, individual dinosaur species were evolving and becoming extinct for various reasons. The unusually massive extinction near the end of the Cretaceous period exterminated the last of the dinosaurs, the flying reptiles, and the large swimming reptiles, as well as many other marine animals. There is now widespread evidence that a meteorite impact was likely at least the partial cause for this extinction. Other factors such as extensive release of volcanic gases, climatic cooling, sea-level change, low reproduction rates, poison gases from a comet, or changes in the Earth's orbit or magnetic field may have contributed to this extinction event.